“I will be in Huntsville, Alabama on Saturday night to support Luther Strange for Senate. ‘Big Luther’ is a great guy who gets things done!” Trump wrote.

Trump’s unexpected move sets the stage for a showdown between the president and his recently departed chief strategist, Steve Bannon, who is all-in for Moore. Bannon has cast the Alabama race as an-important clash between grass-roots conservatives and the Washington establishment — and a test for whether other incumbent senators can be successfully challenged by insurgents in 2018.

In response, Senate Majority Leader Mitch McConnell and other incumbent senators — including Strange himself — have leaned on the president for more help.